I'm writing a story with a little bit of alternate history and my understanding is that East Slavic Mythology dates at least to Kyivan Rus' in 980~'s CE; which predates all of the current day modern states of Russia, Ukraine, Belarus, and so on.
As I also understand it, while the book "Russian Folk Tales" was published by a Russian in Russia in the 1800s, this like Grimm's Fairy Tales, is a compilation of stories, folklore, and myth that dates back to older tales that had been passed down, in verbal or written tradition for hundreds of years. All without getting into metanarratives and Carl Jung's symbology/collective unconscious or Joseph Campbell's Hero of a Thousand Faces but for context that's what's shaping my thinking here where it seems like to my mind characters like Koschei, the Baba Yaga, the Firebird, the Bogatyrs and other heroes of Slavic mythology; just because the modern Russian state widely used these characters and symbols as a part of their cultural heritage doesn't mean it's also not a part of Belarus's or Ukraine's cultural heritage.
I know that when I try to google "Ukrainian folklore" I tend to just end up at tvtropes directed to "East Slavic Mythology" which directs me to Russian folklore; so to my mind these are basically the same or overlapping mythologies? Am I broadly right or am I very wrong?
I understand also that due to current/recent events this might also be a POLITICAL question, but for the purposes of my story I feel like I have good intentions in having some of these previously widely seen as Russian characters as having more Ukrainian or broadly slavic characteristics to them; a sort of reclamation so to speak, but I am neither Russian nor Ukrainian and at the very least I want to know where factually what does the actual academic perspective imply?
So to sorta repeat my thinking goes like this; as most of "Russian" folklore arguably predates modern Russia; it is then arguable to suggest that these characters and stories are part of a broader heritage common to all east slavic cultures? (And maybe South Slavic too but that's even more outside of my ability to even ask insightful questions about!)
e to add: "Slavic" is in quotes as I mainly wish to talk about what are pre-modern cultures and peoples and slavic is a very broad cultural and geographical group in general with a lot of political and historical baggage, so the quotes are mainly just to indicate I am talking in VERY BROAD generalizations, which I hope is taken as intended for the purposes of my sorta question.
'Russian Folk Tales' as compiled by Alexander Afanasyev includes versions recorded in Byelorussian and Ukrainian, too. As you are probably aware, in the 1800s the three languages and cultures were considered components of an overarching 'triune'/pan-'Russian' culture. Frankly, I don't think any serious scholar calls it Russian folklore in the 21st century - or, for that matter, Ukrainian, which would be as anachronistic.
Note that quite a lot of what was traditionally considered part of East Slavic mythology may be erudite inventions dating back to the19th century or even later so you'll need to tread carefully and treat these on a case per case basis.
ETA: I'm not really sure it's only the modern Russian state that uses these as part of their cultural heritage. The three states do. But this is more of a political topic and I don't see how one can have a meaningful discussion about the way these states fight over their shared heritage without breaching this subreddit's 20-years rule.